Gene editing that is specific for a predetermined site can be done with the target-guided nuclease Cas9 and polynucleotide repair methods. Using the target-guided Cas9 endonuclease, both strands of a double stranded DNA can be cut near a target site to create a double-strand break.
The target specificity of Cas9 is determined by a guide molecule, which complexes Cas9 to the polynucleotide target. Polynucleotide target sequences, typically 17-20 bases in length, must be flanked by a 3′ protospacer-adjacent motif (PAM). The structure of PAM is determined by the species of bacteria from which the Cas9 was derived. Fortuitously, suitable target sequences containing a PAM can be found in most genes of interest in most species. In one variation, the guide molecule can be made as a single RNA strand that has a sequence complementary to the target, which is attached to a bacterially-derived crispr-tracr RNA sequence that complexes Cas9.
In some modalities, after forming a double-strand break in dsDNA at a specific site, the break can be repaired to achieve editing of the DNA. A double-strand break can be repaired by non-homologous end joining (NHEJ) to generate random insertions and deletions. A double-strand break can also be repaired by homology-directed repair (HDR) using an exogenous DNA template to generate controlled insertions, deletions, and substitutions.
A major drawback of gene editing with Cas9 is that the guide molecule may have limited effectiveness for a target polynucleotide. The specificity and activity of a guide molecule can be unpredictable. Guide molecules for Cas9 editing can vary widely in effectiveness, and some guides that otherwise follow the structural scheme can be ineffective.
A further drawback of gene editing with Cas9 is that the guide molecule may lack selectivity for a target allele. Variations in the genome can contribute to disease conditions. Some alleles related to disease phenotypes have been identified in medical genetics. The inability to target particular alleles is a significant drawback of current methods for of gene editing.
Other drawbacks of gene editing with CRISPR-Cas systems include the occurrence of off-target mutations.
What is needed are stable and effective guide molecules for gene editing, as well as compositions and methods for use in treating disease.
There is an urgent need for new molecules for guiding gene editing with Cas9, and for allele selectivity and reduced off target activity.